Caller ID, or Caller Identification Service, is a telecommunications service which allows the recipient of a call (i.e., the called party) to identify the party originating a call (i.e., the calling party) before answering the telephone. Conventionally, the information provided to the called party includes the date and time of the call and the caller's telephone number. Caller ID service provides several benefits. First, the identification of the calling party allows the called party to screen an incoming call before its completion, thereby allowing the called party to decide whether or not it is desirable to answer the call. In this way, nuisance calls, such as solicitation, advertising or harassment calls, can be avoided.
Prior to the implementation of Signaling System 7, or simply SS7, identifying information of a calling party making a call was unknown beyond the first telephone company switching system that handled the calling party's telephone service. SS7 signaling technology made practical passing forward a calling party's telephone number to the destination of the call.
In operation, essentially, a data message, preceded by a channel seizure signal, is sent in conjunction with the ringing signal from the central office to the called party during a silent interval after the first 20 Hz, 2 second ringing phase. A detailed description of the method and apparatus for sending the data message may be found in U.S. Pat. No. 4,551,581 issued to Doughty on Nov. 5, 1985.
A customer who subscribes to the Caller ID service has a piece of equipment, known as a caller ID box, located at the customer premises. When the customer receives a call, information is transmitted from the customer's local switch to the caller ID box between the first and second rings. The information transmitted is known as the CPN, and identifies the telephone number associated with the line originating the call, if the CPN is not coded as "presentation restricted." In addition, the date and time of the call are transmitted to the caller ID box during the same interval of time, between the first and second rings. When a call originates from a switch other than the local switch serving the caller ID customer, the CPN should be passed from the originating switch to any intermediate switches, and from those intermediate switches to the customer's terminating switch. This CPN is passed as a parameter in a call setup message called an Initial Address Message (IAM) which is carried over SS7 Networks. If the calling party designates a call as "private," the CPN should still be passed with the coding of "presentation restricted." At the terminating switch, if the received CPN is encoded as "presentation restricted," the switch signals to the caller ID box that the call has been marked "private" or "anonymous."
While the ANSI standards for SS7 identify the CPN as an optional parameter in the IAM, an FCC document, "Rules and Policies Regarding Calling Number Identification Service--Caller ID," CC Docket No. 91-281, requires that "common carriers using Common Channel Signaling System 7 (SS7) and subscribing to or offering any service based on SS7 functionality must transmit the calling party number parameter and its associated privacy indicator on an interstate call to connecting carriers." Despite this FCC mandate, there are recently documented cases in which the CPN was not being transmitted, and, in particular, where AT&T was not receiving the CPN from switches owned by Local Exchange Carriers (LECs). As noted above however, the FCC mandate applies only to common carriers. If a privately-owned PBX is connected to a LEC switch via a Primary Rate Interface, the FCC does not mandate that the CPN be passed from the PBX to the LEC. In that case, the LEC can not pass the CPN to a connecting carrier, even if it does have SS7 connectivity. Additional possibilities as to when the CPN may not be passed are described below.
Prior to the deployment of SS7 signaling, Multi-Frequency (MF) signaling, an earlier form of signaling that does not have the capability to pass CPN between switching systems, was used to set up calls between switching systems. Some switching systems have not yet been converted from MF to SS7 signaling. Therefore, in some situations one or more legs of a call may be set up using SS7 signaling, while other legs may use MF signaling. To allow backward compatibility, an interworking function was defined to allow calls to be set up using SS7, where available, even if SS7 is not fully deployed on every switch carrying a leg of the call. However, whenever a call progresses to a point at which MF, rather than SS7 signaling occurs, some information is lost. Of particular importance for the present invention is the loss of the CPN information. To identify when a call entering a switch via SS7 signaling has encountered MF interworking at some point prior to the current leg, the "Forward Call Indicators" parameter in the IAM is coded to indicate that MF interworking was encountered.
There are situations where, due to technical reasons (e.g., MF interworking), due to policy reasons (e.g., a privately-owned PBX owner choosing not to pass CPN), or, due to equipment or provisioning problems at an originating or intermediate switch, the CPN is not delivered, or can not be delivered. Various problems which could result in a failed transmission of the CPN include: provisioning errors that occur when adding new trunk sub-groups (TSGs), new switches, new SS7 signaling capabilities in existing switches, new software, or other simple human error made in an existing switch during a provisioning change. In any event, if one of these problems causes a failed delivery of the CPN to the terminating switch, the caller ID customer will not receive information for all calls originating from, or connected through, a problematic switch or traversing a problematic TSG. From the standpoint of a caller ID subscriber, the caller ID service provides its advantages only if it is consistent and reliable for a large majority of incoming calls. In addition, the universal delivery of CPN greatly enhances the ability of law enforcement to trace calls when criminal activity is involved. It is therefore desirable to maximize the number of cases in which a CPN is delivered.